Re: LF: RE: Analog oscillators

["James Moritz" <james.moritz@btopenworld.com>]

Dear Stefan, LF Group,

In my experience, the oscillator circuit topology is usually less important 
than the component values used, the type of  components, setting the bias 
current in the transistor, etc.
> It is a bit disapointing. The oscillator is drifting several 100 Hz when 
> looking to the divide by 4 side, i.e. the PA output i.e. close to 475 kHz.
> I used 9 turns on a FT37-43.
I suspect this inductor may be the problem. The temperature coefficient of 
the permeability of ferrite is high, so the inductance will drift a lot with 
temperature. Also, the Q of ferrite cored inductors is usually rather low, 
with the effect of  requiring higher transistor gain to make it oscillate, 
and greater loading of the resonant circuit by the transistor as a result. I 
suggest using a Txx-2 iron dust core insted of the -43 ferrite. It will need 
many more turns, but the inductance will be much more stable and the Q much 
higher. The higher Q will enable you to reduce the bias current in the 
oscillator transistor, and/or increase the shunting capacitor values. This 
will reduce the loading caused by the transistor.
Another way to increase stability is to reduce the operating frequency - an 
oscillator at 475kHz or 950kHz will tend to be more stable because the 
larger fixed capacitance values will mean the junction capacitance of the 
transistor will have less effect.
Cheers, Jim Moritz
73 de M0BMU